Measuring Tennis Racket Forces

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Thomas Haff, physics teacher at Issaquah High School in Issaquah, Washington, likes to assign inquiry-based projects to get his students to use what they have learned. The problem presented to students Kevin Bell and Christopher Nield was to determine the amount of force a tennis racket imparts on a tennis ball during an interaction. Haff chose this project because it is a typical problem found in many college textbooks.

Kevin and Chris rigged a device that would drop a tennis ball a fixed distance to a stationary tennis racket. They considered measuring the velocity of the ball prior to impact with a Photogate, but decided to keep their first attempt simple. Instead, they assumed that the ball would fall freely (neglecting air resistance), obtain the maximum theoretical speed, and collide with the racket elastically.

All that remained was to determine the length of time the ball would be in contact with the racket. To do this, they laced some copper wire in the strings of the tennis racket and attached the wire to a battery, creating an open circuit. They then wrapped the tennis ball with some bare copper wire. When the ball was in contact with the racket, the circuit would close and current would flow. Finally, a Voltage Probe was connected to the circuit to collect voltage data across a section of wire during a trial. Data from one of their trials are displayed below.

It might be interesting to add a photogate to the experiment to see if the students’ assumptions about the velocity of the ball are valid!